The slightly snarky answer is that your wife should simply slow down how quickly she completes the work and then "poof" she magically gets a raise. If this sounds a little ridiculous -- it should. But in my opinion, hourly billing for a position like this is equally ridiculous. For example, if your wife found an app or spreadsheet that allowed her to do her job twice as fast, why would she ever want to use it? She would get paid half as much and yet her "employer" would get the same value of work (actually the employer would get more value because the work could be done faster).
I would encourage you to completely shift your thinking on this. And it starts with hourly billing. Here are some possible steps to consider.
1) Have your wife have a face-to-face meeting with her friend. "Thank you so much for the opportunity to help you ... I'm curious if there are other things I can take off your full-time assistant hands that would free her up for in-office duties.Or are their seasonal or one-time per year projects that I could take over? Tell me about those and
why they are important to your business? What is involved in those projects/duties." Be looking for projects that can be done ahead of time (asynchronous) and that your wife can take complete ownership of (minimal collaboration with other staff, etc.) - also projects that are really, really important (valuable) to the employer. Just as an example, maybe the employer sends out Holiday cards or fruit baskets to her clients each year. Can I compile the list, provide you with 3 options of different gift baskets, and handle the ordering process. Also ask about projects that have been sitting on the shelf because there isn't enough time: "Is there a big project you've always liked to do, but didn't have time and it is something within my skill-set? Perhaps compiling a list of all the accountants in the county because..."
2) Compile a list of these projects. Provide a brief description of what your wife would do to complete them. Review this with employer/friend and make sure you have an accurate view of the scope of the project. Make a mental note of which projects the employer sounds really excited about. Now make revisions to the list and re-submit it with a flat price listed next to each project. Also include a reminder of the value of each project. "This project is important because of the ability to solicit new clients" etc. Bonus points if you can actually determine a rough numerical ROI of the project.
3)
Ignore how many hours it will take your wife to complete the task. Focus entirely on how much value it will provide the employer. Remember the previous example of getting a new app and doing something twice as fast -- your wife should
not be penalized because of how quickly she can complete a project.
4) As mentioned, next to each project, place a price. This is what you will pay me to complete this project by this deadline.
Do not mention how long you think it will take to complete or anything about hours or hourly rates. The question to the employer is simply: "Is it worth it to you for me to get this done for this price?" If the task is an ongoing one (it sounds like several of her current tasks are monthly/regular), it is ok to provide a monthly fee. But again, this should not be based on her hours spent on the task. "If the employer asks, how long will this take you?" your wife can say, "I'm not exactly sure, but I'm happy to do it for this price. The great thing for you(employer) is that you know exactly what your costs will be. Don't you hate it when someone gives you an estimate for a job and then they say 'we ran into problems, it's going to take twice as long' - that won't happen because my prices are fixed per project."
Of course, this will mean your wife needs to have a really good idea of how long something will take. But she should
NOT try to count up the hours and think "That will probably take me 8 hours, so I should price it at $200". Instead, she should think "This will take about 8 hours, so $240 is the
minimum I'd do this task for -- but she said this is a project that could make her $20,000 so I should price it at $500-1000. That is a fair trade of my time and skills for value I'm providing to her."
This is a critical mindset shift that contract/1099 workers need to have. Do
NOT simply try to be an hourly worker who doesn't get benefits and other perks. Sell yourself as a professional that helps with important initiatives, not as a temp worker who fills in the gaps because the FT assistant is really busy. Now, if your wife can find a faster, more efficient way to complete the task, then everyone wins. She works fewer hours and the client likely gets the project done sooner. If your wife can streamline these tasks over time, then great. And the employer/friend gains a huge benefit and knows
exactly what her costs will be instead of possibly arguing with your wife about how long a task should take.
For example, I have clients pay me $280 for something that takes me 5 minutes. But the way it is set up, they are happy to pay it. In fact, 97% of my clients sign up again each year. That's because it provides something of value to them and they don't care how I do it. I don't have a special skill like a brain surgeon or anything, but I've learned how to do this task very, very efficiently. They don't care, they just love that their price is actually lower than my competitors.
If you find this perspective interesting or infuriating, I would encourage you to read anything by Jonathan Stark. Here are his free resources:
https://jonathanstark.com/free 
